Built-in self-test for light emitting diodes

ABSTRACT

In some examples, a device includes a built-in self-test for detecting a fault between a first light emitting diode (LED) and a second LED. The device includes a first pair of pads configured to connect to the first LED and a second pair of pads configured to connect to the second LED. The built-in self-test is configured to control a first driver and a second driver to turn on, one-by-one, respective pass switches connected to the first and second pairs of pads. The built-in self-test is configured to then determine the first and second forward voltages across the first and second LEDs. The built-in self-test can determine whether the fault exists between the first and second LEDs based on the forward voltages.

TECHNICAL FIELD

This disclosure relates to circuitry for driving light sources such aslight emitting devices.

BACKGROUND

Driver circuitry may operate, or drive, one or more light sources, suchas light emitting diodes (LEDs). The driver circuitry may control alight intensity output by an LED by varying an average amount ofelectrical current flowing through the LED and/or by varying an averageforward voltage across the LED. For example, the driver circuitry mayincrease a duty cycle of an electrical current delivered to an LED toincrease a light intensity generated by the LED. Similarly, the drivercircuit may decrease the duty cycle of the electrical current deliveredto an LED to decrease the light intensity generated by the LED. At highswitching frequencies, a human eye may perceive a change in the dutycycle of the electrical current as a change in the brightness orintensity of the light generated by the LED.

Modem automotive LED headlights feature multiple and pixelated lightsources allowing the control of the individual brightness of theactivated pixels or pixel groups. This control enables new lightfunctions such as glare-free high-beam systems, adaptive driving beams,and the projection of symbols. To provide these specific functions, aswell as other functions, the headlight may be capable of providinghigh-resolution light to the entire field of view of the driver.

A headlight can include a matrix array of a large number of LEDs (e.g.,more than sixteen thousand LEDs) with a fine pixel-to-pixel pitch or afine display resolution. The headlight can be constructed using achip-on-chip concept where a second chip including the LEDs is mountedon a first chip including the light source matrix and drivers using ahybridization process. A first company may manufacture the first chip,while a second company may perform the hybridization process. Themanufacture and hybridization processes, especially when performed bydifferent companies, can introduce multiple points of potential failureinto the finished product.

SUMMARY

This disclosure describes techniques for testing light emitting diodes(LEDs) using a built-in self-test. A built-in self-test may includeprocessing circuitry configured to test for a fault between first andsecond LEDs by turning on the first LED and turning off the second LED.The self-test may then be configured to measure the forward voltageacross the first LED and the forward voltage across the second LED.

The details of one or more examples are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages will be apparent from the description and drawings, and fromthe claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual block diagram of a device including drivers forcontrolling the operation of light sources, in accordance with thetechniques of this disclosure.

FIG. 2 is a circuit diagram of amplifiers for controlling pass devicesconnected in series with light emitting diodes (LEDs), in accordancewith the techniques of this disclosure.

FIG. 3 is a circuit diagram of three possible failure modes for a driveror an LED, in accordance with the techniques of this disclosure.

FIG. 4 is a circuit diagram for calibrating a sense resistor, inaccordance with the techniques of this disclosure.

FIGS. 5 and 6 are circuit diagrams for detecting a fault, in accordancewith the techniques of this disclosure.

FIG. 7 is a flow diagram illustrating example techniques for detecting afault, in accordance with the techniques of this disclosure.

FIG. 8 is a circuit diagram for measuring the cathode voltage for twoLEDs, in accordance with the techniques of this disclosure.

FIGS. 9A and 9B are diagrams of two example test patterns that can beimplemented by a built-in self-test, in accordance with the techniquesof this disclosure.

FIG. 10 is a circuit diagram of a device including a differentialanalog-to-digital converter for measuring the forward voltage across anLED, in accordance with the techniques of this disclosure.

FIG. 11 is a flow diagram illustrating example techniques for detectinga fault between two LEDs, in accordance with the techniques of thisdisclosure.

DETAILED DESCRIPTION

This disclosure describes devices, methods, and techniques for testinglight sources using a built-in self-test (BIST). The BIST may beconfigured to detect a fault on a light source, across a pair of pads,and/or in a driver configured to turn the light source. The BIST may beconfigured to test the driver before and/or after the light source isconnected to the driver (e.g., before hybridization). For example, theBIST may be configured to perform a front-end test of the functionalityof each current source (e.g., each driver) before a hybridizationprocess.

After the hybridization process is complete, every light emitting diode(LED) may be connected to (e.g., soldered on) a dedicated currentsource. Therefore, each cathode (for a low-side configuration) or eachanode (for a high-side configuration) may no longer be accessible, whichcan impede testability, field return analysis, and self-diagnosisfunctionality. For example, a BIST of this disclosure may be configuredto check the quality of the assembly and distinguish whether a fault hasoccurred inside an LED chip, inside the current source driver chip, oracross the pads for mounting the LED chip.

FIG. 1 is a conceptual block diagram of a device 100 including drivers130 for controlling the operation of light sources 150, in accordancewith the techniques of this disclosure. In the example of FIG. 1, device100 includes processing circuitry 110, BIST 120, drivers 130, passdevices 140, and pads 160 and 170. LEDs 150 and power source 180 areoptional elements that may be part of device 100 in some examples.Additionally or alternatively, LEDs 150 and/or power source 180 may beexternal components that can be connected, attached, and/or mounted ondevice 100. In some examples, device 100 may be a lighting device for avehicle, a building, and/or any other system that includes a lightingdevice.

Processing circuitry 110 may be configured to control drivers 130 tooperate LEDs 150. For example, processing circuitry 110 may beconfigured to cause one or more of drivers 130 to turn on one or morerespective pass devices 140 to turn on one or more respective LEDs 150.For example, processing circuitry 110 can deliver a low-power controlsignal to drivers 130, which can generate a higher-power driver signalbased on the control signal. Processing circuitry 110 may be configuredto also receive a sensed signal indicating a forward voltage across oneof LEDs or at a respective pair of pads 160 and 170.

Processing circuitry 110 may include any suitable arrangement ofhardware, software, firmware, or any combination thereof, to perform thetechniques attributed to processing circuitry 110 herein. Examples ofprocessing circuitry 110 include any one or more microprocessors,digital signal processors (DSPs), application specific integratedcircuits (ASICs), field programmable gate arrays (FPGAs), or any otherequivalent integrated or discrete logic circuitry, as well as anycombinations of such components. When processing circuitry 110 includessoftware or firmware, processing circuitry 110 further includes anyhardware for storing and executing the software or firmware, such as oneor more processors or processing units. In examples in which device 100is mounted on a vehicle, processing circuitry 110 may be implemented bya headlight controller.

In general, a processing unit may include one or more microprocessors,DSPs, ASICs, FPGAs, or any other equivalent integrated or discrete logiccircuitry, as well as any combinations of such components. Although notshown in FIG. 1, processing circuitry 110 may include a memoryconfigured to store data. The memory may include any volatile ornon-volatile media, such as a random access memory (RAM), read onlymemory (ROM), non-volatile RAM (NVRAM), electrically erasableprogrammable ROM (EEPROM), flash memory, and the like. In some examples,the memory may be external to processing circuitry 110 (e.g., may beexternal to a package in which processing circuitry 110 is housed).

Drivers 130 may be configured to drive pass devices 140 based on signalsreceived from processing circuitry 110. Drivers 130 can includeamplifiers and/or level shifters for generating relatively high-powersignals based on low-power signals received from processing circuitry110. For example, drivers 130 may include a first driver configured todrive a first pass device and a second driver configured to drive asecond pass device, where each pass device is configured to connect toan LED. Additional example details of drivers for light emitting diodescan be found in commonly assigned U.S. Pat. No. 9,930,748, entitled“Synchronization for Light-Source Driver Circuitry,” issued on Mar. 27,2018, and commonly assigned U.S. Pat. No. 10,297,190, entitled “PixelSelection Method for a Light-Source Matrix Driver,” issued on May 21,2019, the entire contents of which are incorporated herein by reference.

Pass devices 140 may be operated as switches that turn on and off.Additionally or alternatively, pass devices 140 may be operated asanalog devices that can operate in a linear mode. In some examples,drivers 130 may be configured to drive pass devices 140 to increase ordecrease the current level conducted by pass devices 140.

LEDs 150 may include two or more light sources such as light-emittingdiodes (LEDs) or any other suitable light sources. Although described inthe context of LEDs, the techniques of this disclosure also can apply toother light sources. LEDs 150 may be arrayed in a matrix or gridformation of columns and rows, and each light source may be a pixel. Insome examples, LEDs 150 may include sixteen thousand and three hundred,and eighty-four light sources that are arrayed in a grid of one hundredand twenty-eight light sources by one hundred and twenty-eight lightsources. Alternatively, LEDs 150 may include one thousand andtwenty-four light sources that are arrayed in a grid of thirty-two lightsources by thirty-two light sources.

Each of LEDs 150 may be driven by a respective one of pass devices 140.In some examples, each LED may be connected in series with a respectivepass device between two power rails (e.g., power source 180). LEDs 150may be part of an external device that can be mounted on andelectrically connected to pads 160 and 170. Pads 160 may connect to thecathodes of LEDs 150, and pads 170 may connect to the anodes of LEDs150. Alternatively, pads 160 may connect to the anodes of LEDs 150, andpads 170 may connect to the cathodes of LEDs 150, and power source 180can be connected to pass devices 140. LEDs 150 may be connected betweenpower source 180 and pass devices 140 as shown in FIG. 1, or thearrangement of pass devices 140 and LEDs 150 may be reversed such thatsuch that pass devices 140 may be connected between power source 180 andLEDs 150 (see, e.g., FIG. 2).

Power source 180 may be configured to supply electrical power to LEDs150 via pads 170 and 182. Power source 180 may include a battery oranother power storage unit, a connection to mains power, and/or one ormore power conversion circuits. Power source 180 may be external todevice 100 in some examples, but in other examples, power source 180 maybe a part of device 100. Device 100 may include one or more pads 182 toallow for power source 180 to mount on and electrically connect todevice 100.

External device 192 can connect to diagnostic interface 190 tocommunicate with processing circuitry 110 and BIST 120. For example,processing circuitry 110 may be configured to transmit data related tothe operation of device 100 to external device 192. In some examples,BIST 120 may receive a command signal (e.g., one or more bits) fromexternal device 192 and may be configured to perform a built-inself-test in response to receiving the command signal. Additionally oralternatively, BIST 120 may be configured to perform a built-inself-test at the time of manufacture and/or at predefined timeintervals. BIST 120 may be configured to perform a self-test before orafter a hybridization process is complete (e.g., front-end or back-end),after the combined device is soldered to a printed circuit board (PCB),and/or when the device is operational in an end-use system (e.g., anautomobile).

Testing for faults on LEDs 150 or on drivers 130 may be difficult whereLEDs 150 are mounted on drivers 130. Where LEDs 150 are mounted ondrivers 130, the node between an LED and a respective driver may behidden or covered by the LED chip. Because some of the nodes areinaccessible, a test probe may not be able to reach these nodes tomeasure a current or voltage.

Analog testing equipment can be used to measure the forward voltagesacross each LED or at a pair of pads, but the measurement time is verylong for a matrix with thousands of LEDs. Analog testing becomesprohibitively time-consuming and costly as the number of LEDs in amatrix steadily increases.

It is important to check the functioning of the LEDs and drivers toensure that the final LED device will operate properly. Testing atseveral points in the manufacture and assembly processes allows for theprompt detection of faults. Testing at intermediate points also allowsfor determining the type and position of each fault, which can be usedto troubleshoot and optimize the manufacture and assembly processes.

In accordance with the techniques of this disclosure, BIST 120 may beconfigured to detect a fault on drivers 130, pass devices 140, and/orLEDs 150 by separately testing each of drivers 130 or each of LEDs 150.To separately test a first driver or a first LED, BIST 120 may beconfigured to cause the first driver to turn on a first pass device thatis configured to connect to the first LED. BIST 120 may be configured tothen determine the voltage levels at the respective pair of pads 160 and170 while the first pass device is activated. BIST 120 may be configuredto separately test each of LEDs 150 one-by-one after LEDs 150 have beenconnected to pads 160 and 170. Based on the voltage levels, BIST 120 candetermine whether a fault exists on the first LED or the first driver.

In some examples, BIST 120 may also be configured to determine thevoltage across each of drivers 130 one-by-one. Based on the voltageacross a driver, BIST 120 may be configured to determine whether a faultexists on the driver and/or whether a fault exists on the respectiveLED. BIST 120 may be configured to separately test each of drivers 130one-by-one before and/or after LEDs 150 are connected to pads 160 and170.

While performing the built-in self-test, BIST 120 can store a count offaults to memory 112. For example, BIST 120 may be configured toincrement a counter in response to determining each fault. There may bemultiple counters to record multiple types of fault. BIST 120 can storedata indicating different categories of faults, such as short-circuitfaults, open-circuit faults, LED faults, driver faults, etc. to memory112. BIST 120 may be configured to store the positions and/or indices ofeach LED or driver that has a fault. BIST 120 can also store datarelating to the forward voltages measured by processing circuitry 110.BIST 120 may be configured to output the data relating to the built-inself-test to external device 192.

Processing circuitry 110 may be configured to perform a remedial actionin response to detecting a fault. For example, processing circuitry 110may be configured to set a bit indicating the fault to prevent a driverfrom turning on a respective pass device to activate the respective LED.A user may be able to define the action to take in response to one ormore possible faults. Processing circuitry 110 can store the option(s)selected by the user in memory 112, so that processing circuitry 110 cantake the associated options.

BIST 120 may be configured to perform the testing routine based in thedigital domain, which may be quicker than testing in an analog domain.For example, BIST 120 may be configured to test each LED in ten totwenty microseconds. By comparison, an analog testing routine may takeapproximately five to ten milliseconds to test each LED. Thus, thetesting routine implemented by BIST 120 may be much faster than othertesting methods such as analog testing methods.

In addition, the testing routine implemented by BIST 120 includes avoltage measurement at two points. One of the two measurement points maybe the positive power source or the negative power source, which aremore easily accessible than the node between an LED and a driver. BIST120 may be configured to access the node between an LED and a driverusing a connection network, which removes the need for manual connectionto an internal node.

FIG. 2 is a circuit diagram of amplifiers 230A-230C for controlling passdevices 240A-240C connected in series with LEDs 250A-250C, in accordancewith the techniques of this disclosure. A device of this disclosure mayinclude processing circuitry 210, amplifiers 230A-230C, pass devices240A-240C, cathode pads 260A-260C, anode pads 270A-270C, and powersupply pad 282. In some examples, the device may also include LEDs250A-250C and/or one or more power sources connected to cathode pads260A-260C and/or and power supply pad 282.

Processing circuitry 210 may include digital logic configured togenerate and deliver lower-power control signals to amplifiers230A-230C. In some examples, each driver may include separate digitallogic, rather than a single centralized block of processing circuitry210. Additionally or alternatively, the device may include multipleblocks of digital logic, where each block generates control signals formore than one driver. The term “driver” as used in the description ofFIG. 2 includes one of amplifiers 230A-230C and a respective one of passdevices 240A-240C. For example, a first driver can include amplifier230A and pass device 240A, a second driver can include amplifier 230Band pass device 240B, and a third driver can include amplifier 230C andpass device 240C. Alternatively, the term driver could also include thedigital logic for generating control signals.

Amplifiers 230A-230C may be configured to generate higher-power driversignals (e.g., by amplifying lower-power control signals) to drive passdevices 240A-240C. Amplifiers 230A-230C can deliver the driver signalsto the control terminals (e.g., gate or base terminals) of pass devices240A-240C. For example, amplifier 230A can activate pass device 240A bydelivering a driver signal to the gate of pass device 240A. Amplifiers230A-230C may be configured to also adjust the current flowing throughpass devices 240A-240C by operating pass devices 240A-240C as analogdevices, rather than operating pass devices 240A-240C as digital deviceswith only two states (on and off).

Pass devices 240A-240C may comprise, but are not limited to, any type offield-effect transistor (FET), a bipolar junction transistor (BJT), aninsulated-gate bipolar transistor (IGBT), a high-electron-mobilitytransistor (HEMT), and/or another element that uses voltage for itscontrol. Pass devices 240A-240C may include n-type transistors and/orp-type transistors. Pass devices 240A-240C may include elements such assilicon, silicon carbide, and/or gallium nitride. In some examples, passdevices 240A-240C include a freewheeling diode connected in parallelwith a transistor to prevent reverse breakdown of the transistor.

LEDs 250A-250C may be formed on a separate chip from processingcircuitry 210, amplifiers 230A-230C, and pass devices 240A-240C. Thechip for LEDs 250A-250C may be attached after the manufacture of thedevice including processing circuitry 210, amplifiers 230A-230C, andpass devices 240A-240C. LEDs 250A-250C can be attached to pads 260A-260Cand 270A-270C during a hybridization process that involves mounting andelectrically connecting LEDs 250A-250C to pads 260A-260C and 270A-270C.Another attachment process may be used to connect a power source to pad282, so that the power source can deliver electrical power to passdevices 240A-240C.

In some examples, there may be a one-to-one ratio of LEDs to passdevices and amplifiers. For example, an LED matrix may includeapproximately sixteen thousand LEDs, where the LED matrix block can bemounted on a device having approximately sixteen thousand pass devicesand approximately sixteen thousand amplifiers. As another example, anLED matrix may include approximately one thousand LEDs, where the LEDmatrix block can be mounted on a device having approximately one passdevices and approximately one amplifiers. Alternatively, there may befewer pass devices and amplifiers than LEDs, and each pass device may beconfigured to control more than one LED.

The configuration shown in FIG. 2 is just one example. The orientationof LEDs 250A-250C can be reversed such that pass devices 240A-240C areconnected to the cathodes of LEDs 250A-250C. In addition, there may beonly one cathode pad instead of multiple cathode pads 260A-260C, wherethe cathodes of LEDs 250A-250C may be connected together within the LEDchip. In examples in which there is a single cathode pad, “pair of pads”refers to one of anode pads 270A-270C and the single cathode pad. Insome examples, LEDs 250A-250C may be part of the same device asamplifiers 230A-230C and pass devices 240A-240C such that pads 260A-260Cand 270A-270C are internal nodes rather than external pads.

A fault may exist on one of LEDs 250A-250C. The fault may be a shortcircuit (low or no resistance) or an open circuit (high or infiniteresistance) across the LED. Processing circuitry 210 may be configuredto detect a short circuit across an LED by determining that the voltageacross the LED when the LED is activated is less than a minimumthreshold voltage. Processing circuitry 210 may be configured to detectan open circuit across an LED by determining that the voltage across theLED when the LED is activated is greater than a maximum thresholdvoltage. The minimum and maximum threshold voltages may be the limits ofan acceptable voltage range for activated LEDs. In response todetermining a fault on an LED, processing circuitry 210 can store theposition or index of the LED and/or increment a counter.

A fault may exist on one of amplifiers 230A-230C or one of pass devices240A-240C. The fault may be a short circuit (low or no resistance) or anopen circuit (high or infinite resistance) across the pass device, orsome other fault on the amplifier. Processing circuitry 210 may beconfigured to detect a short circuit across a pass device by determiningthat the voltage across the pass device is greater than a firstthreshold voltage. Processing circuitry 210 may be configured to detectan open circuit across a pass device by determining that the voltageacross the pass device is activated is less than a second thresholdvoltage.

Processing circuitry 210 may be configured to detect that a fault existson an amplifier or a pass device before LEDs 250A-250C are attached topads 26A-260C and 270A-270C. For example, a sense resistor may beconnected to an anode pad and to a cathode pad, and processing circuitry210 can detect the voltage across the sense resistor. Processingcircuitry 210 may be configured to detect a short circuit across a passdevice by determining that the voltage across the sense resistor whenthe pass device is activated is greater than a first threshold voltage.Processing circuitry 210 may be configured to detect an open circuitacross a pass device by determining that the voltage across the LED whenthe pass device is activated is less than a second threshold voltage.The first and second threshold voltages may comprise an acceptablewindow for the voltage across a sense resistor before a hybridizationprocess, where voltages outside of the acceptable window indicate afault on the pass device.

FIG. 3 is a circuit diagram of three possible failure modes for a driver330A or an LED 350A, in accordance with the techniques of thisdisclosure. Segment 302 includes drivers 330A and 330B, where eachdriver includes one of pass devices 340A and 340B, LEDs 350A and 350B,and pads 360A, 360B, 370A, and 370B. In some examples, LEDs 350A and350B may be part of a separate device from drivers 330A and 330B.

Drivers 330A and 330B are depicted as current sources because turning ona pass device may allow current to flow through the respective LED. Eachof drivers 330A and 330B may include an amplifier configured to drive apass device, as shown in FIG. 2. When the pass device is turned on,current can flow through the pass device and the LED from power source380 to power source 384. When the pass device is turned off, the currentsource IALi can sink or source current. The current source IALi may bepart of driver 330A, and the current source IALj may be part of driver330B.

A semiconductor die including LEDs 350A and 350B may be mounted on pads360A, 360B, 370A, and 370B during a hybridization process. A singlesemiconductor die or semiconductor substrate may include all of theelements shown in FIG. 3, except LEDs 350A and 350B and power sources380 and 384. In examples in which there is a single anode pad instead ofmultiple pads 370A and 370B, the anode terminals of LEDs 350A and 350Bcan be connected to the single anode pad during the hybridizationprocess. In examples in which there is a single anode pad, each ofcathode pads 360A and 360B may be paired with the single anode pad.

Resistances 322, 324, and 326 represent possible faults within segment302. Resistance 322 represents a possible short-circuit fault oropen-circuit fault across pads 360A and 370A (before hybridization) oracross LED 350A (after hybridization). An open circuit is expectedacross pads 360A and 370A before hybridization. A BIST may be configuredto detect a short-circuit fault across pads 360A and 370A or across LED350A by determining that the forward voltage across pads 360A and 370Ais less than a minimum threshold voltage. A BIST may be configured todetect an open-circuit fault across pads 360A and 370A or across LED350A by determining that the forward voltage across pads 360A and 370Ais greater than a maximum threshold voltage.

Resistance 324 represents a possible short-circuit fault or open-circuitfault between pads 360A and 360B (before or after hybridization) orbetween the anodes of LEDs 350A and 350B (after hybridization). A BISTmay be configured to detect a fault between pads 360A and 360B based onthe forward voltage across LED 350A and the forward voltage across LED350B. As shown in FIG. 3, LEDs 350A and 350B are adjacent LEDs, but insome examples a fault can be detected between two non-adjacent LEDs.

Resistance 326 represents a possible short-circuit fault or open-circuitfault across driver 330A before or after a hybridization process occursto attach LEDs 350A and 350B. A BIST may be configured to detect ashort-circuit fault across driver 330A by determining that the voltageacross driver 330A is less than a lower threshold voltage. A BIST may beconfigured to detect an open-circuit fault across driver 330A bydetermining that the voltage across driver 330A is greater than an upperthreshold voltage.

A short circuit can be defined as resistance less than a minimumthreshold resistance, not necessarily zero resistance. Thus, the voltageacross a short circuit may be zero or a small, nonzero voltage. An opencircuit can be defined as resistance greater than a maximum thresholdresistance, not necessarily infinite resistance. The voltage across anopen circuit may be infinite or a large, finite voltage.

Power sources 380 and 384 may be part of another external device. Forexample, pads 382 and 386 may be mounted on a PCB that includesconnections to power sources 380 and 384. In some examples, each pad 382and 386 may include multiple separate pads for parallel connections to,e.g., bond wires. Power sources 380 and 384 may include power rails forpositive and negative power sources such as VDDP and a reference groundpower source. Power sources 380 and 384 may include a power supply suchas a battery or a connection to the mains power supply.

FIG. 4 is a circuit diagram for calibrating a sense resistor 424, inaccordance with the techniques of this disclosure. Logic 410 may beconfigured to calibrate sense resistor 424 for front-end testing (e.g.,before hybridization) in order to reduce or eliminate the spread of theresistance. Calibration of sense resistor 424 may provide a moreaccurate measurement of the absolute value of the current flowingthrough each of the LEDs in segment 402. Logic 410 may be configured tocalibrate sense resistor 424 by varying the resistance of trimmableresistance 426.

Logic 410 may be configured to determine a trimming coefficient thatrepresents the resistance of trimmable resistance 426. Logic 410 canstore the trimming coefficient in coefficient storage 412, which mayinclude one-time programmable memory, flash, or electrically erasableprogrammable read-only memory. Logic 410 may be configured to use thestored trimming coefficient in the application field in addition tofront-end testing. Trimming is optional; alternatives include anexternal resistance or an untrimmed internal resistance. In someexamples, the device may include an internal sense resistance, alongwith pads for connecting an optional external sense resistor.Alternatively, the device may include pads for an external resistorwithout any internal sense resistor. An external resistance may increasethe number of components on an application board (e.g., PCB). Using anuntrimmed internal resistance can result in a spread of the actualresistance value, possibly introducing imprecision in voltage sensing.Therefore, the resulting Gaussian distribution of the resistance couldhave a mean value that is offset from expected value.

Connection network 432 includes switches S₁, S_(N), and S_(R) forselecting and sensing various nodes in segment 402. To calibrate senseresistor 424, calibration routine 414 may be configured to open switchesS₁ to S_(N) and close switch S_(R) to connect sense resistor 424 andtrimmable resistance 426 to test pad 438. Current source 436 can forcean external current from power source 480 through sense resistor 424 andtrimmable resistance 426. Differential analog-to-digital converter (ADC)422 can convert the voltage across sense resistor 424 to a digitalnumber. Calibration routine 414 may be configured to perform acalibration routine by changing the equivalent resistance of trimmableresistance 426. Calibration routine 414 may be configured to store thetrimming coefficient to coefficient storage 412. Additional exampledetails of trimming can be found in commonly assigned U.S. Pat. No.10,069,399, entitled “Selecting an Aspect Ratio for Current Monitoring.”issued on Sep. 4, 2018, the entire contents of which are incorporatedherein by reference.

Although FIGS. 4-6 and 10 depict a single ADC, in some examples a devicemay include more than one ADC to allow for testing the LEDs in parallel.For example, a device may include four ADCs, where each ADC isconfigured to test a subset (e.g., a quadrant) of the LEDs in thedevice. A device with multiple ADCs may be configured to performmultiple test routines in parallel, where each test routine uses one ofthe ADCs.

FIGS. 5 and 6 are circuit diagrams for detecting a fault, in accordancewith the techniques of this disclosure. The testing routine setup shownin FIG. 5 allows for logic 510 to read the forward voltage of each LED.The testing routine shown in FIG. 5 can be used both in front-endtesting (e.g., before hybridization) as well as back-end testing,end-of-line testing, and application field testing. Logic 510 may beconfigured to evaluate if an LED has been connected over a pair of padsor if an open circuit or a short circuit is instead present across thepair of pads.

To enable the routine, finite state machine (FSM) 514 may be configuredto control connection network 532 to disconnect ADC 522 from senseresistor 524 by opening switch S_(R). FSM 514 may be configured to thenconnect ADC 522 to test pad 538 by closing switch S_(FSM). Test pad 538is shared by all of the LEDs in segment 502, and logic 510 may beconfigured to connect the LEDs to test pad 538 one-by-one. FSM 514 canturn on the LEDs by activating drivers one-by-one, and ADC 522 cansequentially sense the forward voltage of the LED by controllingswitches S₁ to S_(N) that are connected to ADC 522. Alternatively. FSM514 can turn on all of the LEDs in parallel, but this approach willresult in higher power dissipation. FSM 514 can also turn on a batch ofLEDs at once and proceed to determine the forward voltages across eachof the LEDs.

ADC 522 can convert the forward voltage across current source 530A to adigital number. An internal logic calculator inside logic 510 may beconfigured to evaluate if the digital number is greater than a maximumthreshold voltage or less than a minimum threshold voltage. Currentsource 530A is an example of a driver (e.g., an amplifier and a passdevice) for controlling an LED.

In response to determining that the voltage across current source 530Ais greater than an upper threshold voltage, logic 510 may be configuredto store the corresponding pixel index and report an open-circuitcondition. In response to determining that the voltage across currentsource 530A is less than a lower threshold voltage, logic 510 may beconfigured to store the corresponding pixel index and report ashort-circuit condition. The upper and lower threshold voltages maycomprise an acceptable window for the voltage across current source530A. Logic 510 may be configured to store numbers of each fail count toregisters 512. Logic 510 may then report the fail counts and positionsof each fault to an external device via diagnostic interface 590.

Logic 510 may be configured to run the testing routine in theapplication field (e.g., during normal operation) by enabling thetesting routine at the start-up of the device. Logic 510 may beconfigured to perform the testing routine at regular and/or predefinedintervals, and/or logic 510 may be configured to perform the testingroutine in response to receiving a command signal, for example, from anexternal device via diagnostic interface 590. Logic 510 may beconfigured to transmit the results of the testing routine via diagnosticinterface 590 to an external device to inform the user of failed pixelsand fault counts. The testing results can allow the user to screen fordefective parts and to evaluate the outcome of the hybridizationprocess.

FIG. 6 depicts a testing routine to measure the output current duringfront-end or back-end. For front-end testing (e.g., before hybridizationprocess), logic 610 may be configured to check functionality of eachcurrent source in a reasonable time and in order to have statisticaldata with low effort for testing and for data storage. Logic 610 may beconfigured to test each current source in front-end post-assembly orback-end (e.g., after hybridization process), evaluate impact of theassembly process on the output current and distinguish if a fault ispresent on the LED-attachment side or on the current-source side.Post-assembly testing may be important to localize the position of thefault and also for field return analysis.

Sense resistor 624 may include an internal resistance (e.g., untrimmedor previously trimmed) or an external resistor. Logic 610 may beconfigured to control connection network 632 to cause the current from adriver to flow on sense resistor 624. Logic 610 may be configured totest the drivers one-by-one by connecting sense resistor 624 to a firstdriver, then connecting sense resistor 624 to a second driver, and soon. ADC 622 may be configured to sense the voltage across sense resistor624, which indicates the voltage across the selected LED. ADC 622 canconvert the voltage across sense resistor 622 into a digital number. Thepositive input of ADC 622 is configured to receive a first signalindicating the voltage level at a first end of sense resistor 624 (e.g.,the end connected to power source 680). The negative input of ADC 622 isconfigured to receive a second signal indicating the voltage level at asecond end of sense resistor 624 (e.g., the end connected to test pad638).

To enable the automatic routine, FSM 614 may be configured to connectADC 622 to sense resistor 624 by closing switches S_(FSM) and to connectADC 622 to test pad 638 by closing switch S_(R). FSM 614 may beconfigured to turn on each driver one-by-one (or alternative all at oncein parallel) and sequentially connect each current source to senseresistor 624. The current of the attached driver will flow through thesense resistor 624, and ADC 622 can convert the voltage across senseresistor 624. Based on Ohm's Law and the known resistance of senseresistor 624, logic 610 can determine the current conducted by therespective driver. Logic 610 may be configured to determine whether afault exists on the driver based on whether the determined current fallswithin an acceptable range.

To determine whether a fault exists on a driver, logic 610 may beconfigured to evaluate whether the digital number outputted by ADC 622is outside of an acceptable range, where the acceptable range is boundby a maximum threshold voltage and a minimum threshold voltage. Forexample, in response to determining that the forward voltage across LED650A is greater than a maximum threshold voltage, logic 610 may beconfigured to determine that an open circuit exists across pads 660A and670A. In response to determining that the forward voltage across LED650A is less than a minimum threshold voltage, logic 610 may beconfigured to determine that a short circuit exists across pads 660A and670A. Logic 610 may also be configured to determine whether the cathodeterminal of LED 650A is properly connected to cathode pad 670A, orwhether the connection is missing or impeded.

In response to determining that digital number is outside of theacceptable range, logic 610 may be configured to store the pixel indexand/or an indication of the type of fault. Logic 610 may also beconfigured to store data to coefficient storage 612 such as fault countnumbers, the minimum and maximum voltages (e.g., digital numbersoutputted by ADC 622), and/or the average voltage. The minimum, maximum,and mean voltages may be based on all of the voltages sensed by ADC 622.

Logic 610 may be configured to store testing parameters to a set ofregisters. The testing parameters may include a bit for enabling outputcurrent measurement routine, a bit for enabling VLED forward voltagemeasurement routine, and a bit for enabling sense resistancecalibration. The testing parameters can also include the settling timebetween two consecutive current source selections, the number ofrepetitive conversions on the same current source, high- and low-sidelimit (e.g., maximum and minimum threshold voltages), and maximum,minimum, and average threshold current and voltage values. The testingparameters may also include maximum and minimum pixel coordinate valuesfor testing and one or more counters for the number of detected faults.

Logic 610 may be configured to report all of the stored data to anexternal device via diagnostic interface 690. With all the data storedin diagnostic registers, logic 610 can read-out the maximum, minimum,and mean values of the digital numbers and evaluate the Gaussiandistribution and the process capability index (cpk) without the need tohave statistical data of all the current sources. Logic 610 can performthe testing routine before or after the LEDs are mounted on the currentsources. For example, sense resistor 624 can be connected in parallel toan LED. With an appropriate choice of the resistance value, the currentcan partially or fully bypass the LED and allow the current from thedriver to flow on sense resistor 624. By setting the resistance of senseresistor 624 low enough, logic 610 can measure the current conducted bythe driver by measuring the voltage across the sense resistor 624.

In examples in which logic 610 has identified a fault on an LED, logic610 may be configured to check whether the fault exists on the LEDand/or the respective driver by connecting sense resistor 624 in serieswith the driver. Logic 610 may be able to distinguish between an LEDfault and a driver fault using the output current measurement using theconfiguration shown in FIG. 6.

FIG. 7 is a flow diagram illustrating example techniques for detecting afault, in accordance with the techniques of this disclosure. Thetechniques of FIG. 7 are described with reference to the circuit shownin FIG. 2, although other components may exemplify similar techniques.

In the example of FIG. 7, processing circuitry 210 causes amplifier 230Aof a first driver to turn on pass device 240A (700). Processingcircuitry 210 can deliver a control signal to amplifier 230A, andamplifier 230A can generate a higher-power driver signal based on thecontrol signal. In examples in which LED 250A is mounted on pads 260Aand 270A, processing circuitry 210 can turn on LED 250A by controllingamplifier 230A to activate pass device 240A.

In the example of FIG. 7, processing circuitry 210 determines thevoltage levels at anode pad 270A and at cathode pad 260A (702). To sensethe voltage levels, processing circuitry 210 may be configured tocontrol a connection network of switches to connect a sense resistoracross pads 260A and 270A. A differential ADC may be configured toconvert the voltage across the sense resistor to a digital numberindicative of the voltage across pads 260A and 270A (e.g., the voltagedifference between pads 260A and 270A).

In the example of FIG. 7, processing circuitry 210 determines that afault exists on the driver or on LED 250A based on the voltage levels atpads 260A and 270A (704). In examples in which LED 250A has not yet beenmounted on pads 260A and 270A, processing circuitry 210 may beconfigured to determine that a fault exists on the driver (e.g.,amplifier 230A or pass device 240A) or across the gap between pads 260Aand 270A, which should be an open circuit before LED 250A is attached.

In examples in which LED 250A has been mounted on pads 260A and 270A,processing circuitry 210 may be configured to first determine thevoltage drop across pass device 240A while LED 250A is turned on. Inresponse to detecting that the voltage drop across pass device 240A isoutside of an acceptable range, processing circuitry 210 may beconfigured to then determine the voltage drop across pads 260A and 270Ain order to determine whether the fault exists on pass device 240Aand/or LED 250A. In examples in which processing circuitry 210determines that both voltage levels are outside of the acceptableranges, processing circuitry 210 may be configured to store anindication of two distinct faults.

FIG. 8 is a circuit diagram for measuring the cathode voltage for twoLEDs 850A and 850B, in accordance with the techniques of thisdisclosure. To detect a fault between LEDs 850A and 850B, logic 810 maybe configured to control connection network 832 to connect cathode pads860A and 860B one-by-one to test pad 838. While cathode pad 860A isconnected to test pad 838, for example, current source 836 can sinkcurrent from LED 850A.

Current sensor 834 is configured to measure the current conducted bycurrent source 836. Logic 810 can determine whether an LED-to-LED faultexists based on the current sensed by current sensor 834 while cathodepad 860A is connected to test pad 838 and while cathode pad 860B isconnected to test pad 838. For example, logic 810 may be configured toforce a current through LED 850A (e.g., by activating driver 830A) andchecking whether the current is present at cathode pad 860B. The currentfrom LED 850A should not be present on cathode pad 860B unless there isa relatively low resistance pathway between cathode pads 860A and 860B.

FIGS. 9A and 9B are diagrams of two example test patterns 900A and 900Bthat can be implemented by a BIST, in accordance with the techniques ofthis disclosure. The BIST may be configured to perform automaticallyperform a low-power testing routine using test patterns 900A and 900B.Test patterns 900A and 900B use a complementary chess-board pattern. TheBIST may be configured to detect a specific failure mode using testpatterns 900A and 900B: an LED-to-LED short circuit.

Test patterns 900A and 900B, together with the automatic measurement ofinternal parameters via a differential ADC, allow for fully automaticdetection of LED-to-LED failures (e.g., for verification ofchannel-to-channel crosstalk). For each of test patterns 900A and 900B,the BIST can create a complementary chessboard pattern by setting thestatus of an ith cell to 100% (activated) and setting the status of an(i+1)th cell to 0% (deactivated). The cells may be numbered sequentiallyso that the first cell is adjacent to the second cell, the second cellis adjacent to the third cell, and so on down the row. If the sixteenthcell is at the end of a row, the seventeenth cell is at the beginning ofthe next row. Test pattern 900B is the inverse of test pattern 900A.

For test pattern 900A, the BIST may be configured to turn on a firstsubset of the LEDs. The first subset may include a first LED, a thirdLED, a fifth LED, and so on. For test pattern 900A, the BIST may also beconfigured to turn off a second subset of the LEDs. The second subsetmay include a second LED, a fourth LED, a sixth LED, and so on. Thefirst and second subsets of LEDs may form a chessboard or checkerboardpattern, as shown in FIGS. 9A and 9B. Other test patterns ar possible,such as ring patterns or stripe patterns. For example, a test patterncan allow for sensing a fault between two diagonally positioned LEDs(e.g., different row and different column).

FIG. 10 is a circuit diagram of a device including a differential ADC1022 for measuring the forward voltage across an LED, in accordance withthe techniques of this disclosure. When applying the dedicated lightpattern, logic 1010 may be configured to perform an automatic routineinvolving the measurement of the forward voltage across each LED of anentire matrix using ADC 1022. To enable the automatic routine, logic1010 may be configured to control a connection network 1032 to connectADC 1022 to power source 1080 and to cathode pad 1060A or 1060B of theselected pixel using the switches labeled DIAG_SEL.

Logic 1010 may include an internal FSM configured to automaticallyenable ADC 1022 to convert of the forward voltage of each LED to adigital number. An internal logic calculator may be configured toevaluate whether the forward voltage of the activated LED pixel iswithin an acceptable range for activated LEDs. The internal logiccalculator may also be configured to evaluate whether the forwardvoltage of the deactivated LED pixel is within an acceptable range fordeactivated LEDs. In the example depicted in FIG. 10, LED 1050A is anactivated LED, and LED 1050B is a deactivated LED.

Each of the acceptable ranges may include a respective upper thresholdand a respective lower threshold. The acceptable range for activatedLEDs can include the typical forward voltage for an LED, which may bebetween 1.7 volts and 3.3 volts, depending on the type of LED. The upperthreshold voltage for activated LEDs may be a range from four to fivevolts, such as 4.5 volts. In examples in which LED 1050A includesmultiple LEDs connected in series, the typical voltage may be higherthan 3.3 volts. The acceptable range for deactivated LEDs can includezero volts. Each acceptable range may be broader than the typicalvoltage; for example, the acceptable range for a deactivated LED couldbe negative 0.3 volts to positive 0.6 volts. Thus, to determine whethera fault exists between cathode pads 1060A and 1060B, logic 1010 may beconfigured to determine whether the forward voltage of LED 1050A is lessthan a lower threshold voltage for activated LEDs and determine whetherthe forward voltage of LED 1050B is greater than an upper thresholdvoltage for deactivated LEDs.

In some examples, the expected forward voltage for an activated LED isapproximately three volts above reference ground at the GND node, andthe voltage level of VDDP is four volts above reference ground. Thus,the expected voltage level at cathode pad 1060A is approximately onevolt above reference ground. Logic 1010 may be configured to detect ashort from cathode pad 1060A to VDDP or some other fault in response todetermining that the voltage level at cathode pad 1060A is greater thanan upper threshold voltage, such as two volts above reference ground.Logic 1010 may be configured to detect a short from cathode pad 1060A toreference ground in response to determining that the voltage level atcathode pad 1060A is less than a lower threshold voltage, such as 0.5volts above reference ground.

If the converted values for the forward voltages across LEDs 1050A and1050B are outside the respective acceptable ranges means that a dirtyresistance is placed between adjacent pixels. For example, an opencircuit (e.g., high resistance) may be desirable between cathode pads1060A and 1060B. If the resistance decreases below an acceptable level,the voltages at cathode pads 1060A and 1060B, as measured by ADC 1022,will not be within the acceptable voltage ranges.

Logic 1010 may be configured to store the number of detected faults andthe positions of each fault in a memory. Logic 1010 may be furtherconfigured to report this data to an external device via a diagnosticinterface. Testing for a fault between LEDs can occurred at theback-end, the end-of-line, or in the field. Back-end testing may occurafter a hybridization process is complete, end-of-line testing may occurafter the LED device is fully assembled, and field testing can occurafter the device is installed and operating.

The testing parameters for LED-to-LED testing may include a stored valuefor the type of chessboard pattern, a value for the settling timebetween two consecutive current source selections, the number ofrepetitive conversions on the same current source, high- and low-sidelimit (e.g., maximum and minimum threshold voltages), and maximum,minimum, and average threshold current and voltage values. The testingparameters may also include maximum and minimum pixel coordinate valuesfor testing and one or more counters for the number of detected faults.

FIG. 11 is a flow diagram illustrating example techniques for detectinga fault between two LEDs, in accordance with the techniques of thisdisclosure. The techniques of FIG. 11 are described with reference tothe circuit shown in FIG. 10, although other components may exemplifysimilar techniques.

In the example of FIG. 11, logic 1010 activates LED 1050A (1000) anddeactivates LED 1050B (1002). Logic 1010 can activate LED 1050A bycausing driver 1030A to activate a pass device connected to LED 1050A.Activating this pass device allows current to flow through LED 1050A.Logic 1010 can deactivate LED 1050B by causing driver 1030B todeactivate a pass device connected to LED 1050AB. Activating this passdevice prevents or reduces current from flowing through LED 1050B.

In the example of FIG. 11, logic 1010 determines the forward voltageacross LED 1050A (1004) and the forward voltage across LED 1050B (1006).Logic 1010 can measure the forward voltage across LED 1050A byconnecting ADC 1022 across pads 1060A and 1070A. Logic 1010 may beconfigured to then control connection network 1032 to disconnect ADC1022 from pads 1060A and 1070A and connect ADC 1022 to pads 1060B and1070B, but in other examples the anode pads of adjacent LEDs may not bedirectly connected. In the example of FIG. 10, anode pad 1070A isconnected to anode pad 1070B. ADC 1022 can convert each forward voltageto a digital number that is stored in memory by logic 1010.

In the example of FIG. 11, logic 1010 determines whether a fault existsbetween LEDs 1050A and 1050B based on the forward voltages across LEDs1050A and 1050B (1008). For example, logic 1010 may be configured todetermine whether each of the forward voltages is within an acceptablevoltage range. Logic 1010 may also be configured to measure the voltageacross driver 1030A and/or 1030B in order to determine whether the faultexists across a single LED or across a single driver. In response todetermining that a fault exists between cathode pads 1070A and 1070B,logic 1010 may be configured to increment a fault counter and store thepositions of LEDs 1050A and 1050B.

This disclosure has attributed functionality to processing circuitry 110and 210, BIST 120, and logic 410, 510, 610, 810, and 1010. Processingcircuitry 110 and 210, BIST 120, and logic 410, 510, 610, 810, and 1010may include one or more processors. Processing circuitry 110 and 210,BIST 120, and logic 410, 510, 610, 810, and 1010 may include anycombination of integrated circuitry, discrete logic circuitry, analogcircuitry, such as one or more microprocessors, digital signalprocessors (DSPs), application specific integrated circuits (ASICs),and/or field-programmable gate arrays (FPGAs). In some examples,processing circuitry 110 and 210, BIST 120, and logic 410, 510, 610,810, and 1010 may include multiple components, such as any combinationof one or more microprocessors, one or more DSPs, one or more ASICs, orone or more FPGAs, as well as other discrete or integrated logiccircuitry, and/or analog circuitry.

The techniques described in this disclosure may also be embodied orencoded in an article of manufacture including a non-transitorycomputer-readable storage medium, such as processing circuitry 110 and210, BIST 120, and logic 410, 510, 610, 810, and 1010. Examplenon-transitory computer-readable storage media may include RAM, ROM,programmable ROM (PROM), erasable programmable ROM (EPROM),electronically erasable programmable ROM (EEPROM), flash memory, a harddisk, magnetic media, optical media, or any other computer readablestorage devices or tangible computer readable media. The term“non-transitory” may indicate that the storage medium is not embodied ina carrier wave or a propagated signal. In certain examples, anon-transitory storage medium may store data that can, over time, change(e.g., in RAM or cache).

The following numbered examples demonstrate one or more aspects of thedisclosure.

Example 1. A method for a built-in self-test to detect a fault includescausing a first driver of a set of drivers to turn on a first respectivepass device, wherein each driver of the set of drivers includes a passdevice configured to electrically connect to a respective pad of the atleast one anode pad or the at least one cathode pad, and wherein eachdriver of the set of drivers is configured to control whether arespective LED of the set of LEDs is turned on or off. The method alsoincludes determining a first voltage level at a first anode pad of theat least one anode pad and determining a first voltage level at a firstcathode pad of the at least one cathode pad. The method further includesdetermining that a fault exists on the first driver, across the firstanode pad and the first cathode pad, or on a first LED based on thefirst voltage levels at the first anode pad and the first cathode pad.

Example 2. The method of example 1, further including determining thefirst voltage levels at the first anode pad and the first cathode pad bydetermining a voltage drop across a sense resistor while the senseresistor is connected across the first anode pad and the first cathodepad.

Example 3. The method of example 1 or example 2, further includingseparately testing each driver of the set of drivers one-by-one or eachLED of the set of LEDs one-by-one by at least causing the sense resistorto be connected across a second anode pad of the at least one anode padand a second cathode pad of the at least one cathode pad and determiningsecond voltage levels at the second anode pad and at the second cathodepad while the sense resistor is connected across the second anode padand the second cathode pad.

Example 4. The method of examples 1-3 or any combination thereof,wherein separately testing each driver or each LED one-by-one includescausing a second driver of the set of drivers to turn on a secondrespective pass device.

Example 5. The method of examples 1-4 or any combination thereof,wherein separately testing each driver or each LED one-by-one includesdetermining that a fault exists on the second driver or on a second LEDbased on the second voltage levels at the second anode pad and thesecond cathode pad.

Example 6. The method of examples 1-5 or any combination thereof,wherein separately testing each driver or each LED one-by-one includescontrolling the connection network to connect the sense resistor acrossthe first anode pad and the first cathode pad before determining thefirst voltage levels.

Example 7. The method of examples 1-6 or any combination thereof,wherein separately testing each driver or each LED one-by-one includescontrolling the connection network to disconnect the sense resistor fromthe first anode pad and the first cathode pad before determining thesecond voltage levels.

Example 8. The method of examples 1-7 or any combination thereof,wherein separately testing each driver or each LED one-by-one includescontrolling the connection network to connect the sense resistor acrossthe second anode pad and the second cathode pad before determining thesecond voltage levels.

Example 9. The method of examples 1-8 or any combination thereof,further including outputting, by a differential ADC, a digital numberindicating a voltage drop across the sense resistor to the processingcircuitry.

Example 10. The method of examples 1-9 or any combination thereof,wherein determining that the fault exists is based on the digitalnumber.

Example 11. The method of examples 1-10 or any combination thereof,further including causing a current source to deliver a test current tothe sense resistor while the sense resistor is disconnected from the atleast one anode pad and the at least one cathode pad.

Example 12. The method of examples 1-11 or any combination thereof,further including determining a voltage drop across the sense resistorwhile the test current is being delivered to the sense resistor.

Example 13. The method of examples 1-12 or any combination thereof,further including changing a resistance of the sense resistor based onthe determined voltage drop across the sense resistor.

Example 14. The method of examples 1-13 or any combination thereof,further including changing the resistance of the sense resistor using atrimming coefficient.

Example 15. The method of examples 1-14 or any combination thereof,further including storing the trimming coefficient to a memory.

Example 16. The method of examples 1-15 or any combination thereof,wherein determining that the fault exists includes determining that adifference between the first voltage level at the first anode pad andthe voltage level at the first cathode pad is greater than an upperthreshold.

Example 17. The method of examples 1-16 or any combination thereof,wherein determining that the fault exists includes determining that anopen circuit exists across the first anode pad and the first cathode padin response to determining that the difference between the first voltagelevels is greater than the upper threshold.

Example 18. The method of examples 1-17 or any combination thereof,wherein determining that the fault exists includes determining that adifference between the first voltage level at the first anode pad andthe voltage level at the first cathode pad is less than a lowerthreshold.

Example 19. The method of examples 1-18 or any combination thereof,wherein determining that the fault exists includes determining that ashort circuit exists across the first anode pad and the first cathodepad in response to determining that the difference between the firstvoltage levels is less than the lower threshold.

Example 20. The method of examples 1-19 or any combination thereof,further including storing a count of faults to the memory.

Example 21. The method of examples 1-20 or any combination thereof,further including storing a first count of detected open-circuit faultsto the memory and storing a second count of detected short-circuitfaults to the memory.

Example 22. The method of examples 1-21 or any combination thereof,further including outputting the count of faults to an external devicevia a diagnostic interface.

Example 23. The method of examples 1-22 or any combination thereof,further including determining whether the count of faults is greaterthan a threshold number and outputting the count of faults to theexternal device in response to determining that the count of faults isgreater than the threshold number.

Example 24. The method of examples 1-23 or any combination thereof,further including determining a minimum value of voltage differencesacross each respective anode pad of the at least one anode pad and eachrespective cathode pad of the at least one cathode pad.

Example 25. The method of examples 1-24 or any combination thereof,further including determining a maximum value of voltage differencesacross each respective anode pad of the at least one anode pad and eachrespective cathode pad of the at least one cathode pad.

Example 26. The method of examples 1-25 or any combination thereof,further including determining a mean value of voltage differences acrosseach respective anode pad of the at least one anode pad and eachrespective cathode pad of the at least one cathode pad.

Example 27. The method of examples 1-26 or any combination thereof,further including storing the minimum value, the maximum value, and themean value to a memory.

Example 28. The method of examples 1-27 or any combination thereof,further including outputting the minimum value, the maximum value, andthe mean value to an external device via a diagnostic interface.

Example 29. The method of examples 1-28 or any combination thereof,further including concurrently test a plurality of drivers of the set ofdrivers or a plurality of LEDs of the set of LEDs as a batch by at leastcausing the plurality of drivers to turn on a respective plurality ofpass devices one-by-one, determining a plurality of voltage differencesacross respective pads one-by-one while a pass device of the respectiveplurality of pass devices are turned on, and determining whether anyfaults exist based on the plurality of voltage differences. Theplurality of drivers comprises less than all of the set of drivers, andthe plurality of LEDs comprises less than all of the set of LEDs.

Example 30. The method of examples 1-29 or any combination thereof,further including receiving a signal from an external device via adiagnostic interface and determining whether the fault exists inresponse to receiving the signal from the external device.

Example 31. The method of examples 1-30 or any combination thereof,wherein determining whether the fault exists occurs before and after ahybridization process.

Example 32. The method of examples 1-31 or any combination thereof,further including determining whether the fault exists across the firstanode pad and the first cathode pad before a hybridization process.

Example 33. The method of examples 1-32 or any combination thereof,wherein determining whether the fault exists on the first LED occursafter the hybridization process.

Example 34. The method of examples 1-33 or any combination thereof,wherein the hybridization process includes mounting the device on astructure including the set of LEDs and electrically connecting the setof switches to the set of LEDs.

Example 35. A device includes a built-in self-test for detecting afault, the device further including at least one anode pad configured toconnect to anodes of a set of LEDs and at least one cathode padconfigured to connect to cathodes of the set of LEDs. The device furtherincludes a set of drivers, wherein each driver of the set of driversincludes a pass device configured to electrically connect to arespective pad of the at least one anode pad or the at least one cathodepad, and wherein each driver of the set of drivers is configured tocontrol whether a respective LED of the set of LEDs is turned on or off.The device also includes processing circuitry configured to separatelytest each driver of the set of drivers one-by-one or each LED of the setof LEDs one-by-one by at least causing a first driver of the set ofdrivers to turn on a first respective pass device, determining a firstvoltage level at a first anode pad of the at least one anode pad,determining a first voltage level at a first cathode pad of the at leastone cathode pad, and determining that a fault exists on the firstdriver, across the first anode pad and the first cathode pad, or on afirst LED based on the first voltage levels at the first anode pad andthe first cathode pad.

Example 36. The device of example 35, further including a sense resistorconfigured to be connected to the first anode pad and to the firstcathode pad.

Example 37. The device of example 35 or example 36, further including adifferential ADC configured to measure the voltage across a senseresistor.

Example 38. The device of examples 35-37 or any combination thereof,further including a connection network configured to connect to thefirst anode pad and the first cathode pad.

Example 39. The device of examples 35-38 or any combination thereof,further including a memory configured to store a trimming coefficientfor a sense resistor.

Example 40. The device of examples 35-39 or any combination thereof,further including a memory configured to store a minimum value, amaximum value, and a mean value of voltage differences across each LED,each pair of pads, and/or each driver.

Example 41. The device of examples 35-40 or any combination thereof,wherein the processing circuitry includes a state machine configured tocause the set of drivers to turn on and off pass devices one by one aspart of a self test.

Example 42. The device of examples 35-41 or any combination thereof,wherein the processing circuitry is configured to perform the method ofexamples 1-34 or any combination thereof.

Example 43. A device includes a computer-readable medium havingexecutable instructions stored thereon, configured to be executable byprocessing circuitry for causing the processing circuitry to perform themethod of examples 1-34 or any combination thereof.

Example 44. A system includes means for performing the method ofexamples 1-34 or any combination thereof.

Example 45. A method for a built-in self-test to detect a fault betweena first LED and a second LED of a set of LEDs includes causing a firstdriver of a set of drivers to turn on a first respective pass device.Each driver of the set of drivers includes a pass device configured toelectrically connect to a respective pad of the at least one anode pador the at least one cathode pad, and wherein each driver of the set ofdrivers is configured to control whether a respective LED of the set ofLEDs is turned on or off. The method also includes causing a seconddriver of the set of drivers to control a second respective pass deviceto turn off the second LED, determining a first forward voltage acrossthe first LED, and determining a second forward voltage across thesecond LED. The method further includes determining that a fault existsbetween a first LED and a second LED of the set of LEDs based on thefirst and second forward voltages.

Example 46. The method of example 45, further including determiningwhether the first forward voltage is outside of a first acceptable rangefor activated LEDs.

Example 47. The method of example 45 or example 46, further includingdetermining whether the second forward voltage is outside of a secondacceptable range for deactivated LEDs.

Example 48. The method of examples 45-47 or any combination thereof,wherein determining that the fault exists includes determining that thefirst forward voltage is outside of the first acceptable range.

Example 49. The method of examples 45-48 or any combination thereof,wherein determining that the fault exists includes determining that thesecond forward voltage is outside of the second acceptable range.

Example 50. The method of examples 45-49 or any combination thereof,wherein determining whether the first forward voltage is outside of thefirst acceptable range includes determining that the first forwardvoltage is less than a first threshold level for activated LEDs.

Example 51. The method of examples 45-50 or any combination thereof,wherein determining whether the second forward voltage is outside of thesecond acceptable range includes determining that the second forwardvoltage is greater than a second threshold level for deactivated LEDs.

Example 52. The method of examples 45-51 or any combination thereof,further including outputting, by a differential ADC, a digital numberindicating the first forward voltage across the first LED or indicatingthe second forward voltage across the second LED to the processingcircuitry.

Example 53. The method of examples 45-52 or any combination thereof,further including causing a first input of a differential ADC to receivea first signal indicating a voltage level at an anode pad of the firstpair of pads.

Example 54. The method of examples 45-53 or any combination thereof,further including causing a second input of a differential ADC toreceive a second signal indicating a voltage level at a cathode pad ofthe first pair of pads.

Example 55. The method of examples 45-54 or any combination thereof,further including causing the first input of a differential ADC toreceive a third signal indicating a voltage level at an anode pad of thesecond pair of pads instead of receiving the first signal.

Example 56. The method of examples 45-55 or any combination thereof,further including causing the second input of a differential ADC toreceive a fourth signal indicating a voltage level at a cathode pad ofthe second pair of pads instead of receiving the first signal.

Example 57. The method of examples 45-56 or any combination thereof,further including incrementing a counter in response to determining thatthe fault exists between the first LED and the second LED.

Example 58. The method of examples 45-57 or any combination thereof,further including outputting a value of the counter to an externaldevice via a diagnostic interface.

Example 59. The method of examples 45-58 or any combination thereof,further including storing, to a memory, a position of the first LED anda position for the second LED in response to determining that the faultexists between the first LED and the second LED.

Example 60. The method of examples 45-59 or any combination thereof,further including outputting the position of the first LED and theposition of the second LED to an external device via a diagnosticinterface.

Example 61. The method of examples 45-60 or any combination thereof,further including turning on a first subset of the set of LEDs andturning off a second subset of the set of LEDs.

Example 62. The method of examples 45-61 or any combination thereof,wherein the first and second subsets form a chessboard pattern.

Example 63. The method of examples 45-62 or any combination thereof,wherein the first and second subsets form a ring pattern or a stripedpattern.

Example 64. The method of examples 45-63 or any combination thereof,further including receiving a signal from an external device via adiagnostic interface.

Example 65. The method of examples 45-64 or any combination thereof,further including determining whether the fault exists between the firstLED and the second LED in response to receiving a signal from anexternal device.

Example 66. The method of examples 45-65 or any combination thereof,further including performing the built-in self-test in response toreceiving a signal from an external device.

Example 67. The method of examples 45-66 or any combination thereof,further including determining whether the fault exists between the firstLED and the second LED during a startup phase of the device.

Example 68. The method of examples 45-67 or any combination thereof,wherein the first LED is adjacent to the second LED.

Example 69. The method of examples 45-68 or any combination thereof,wherein the first LED arranged diagonal to the second LED in a matrix.

Example 70. The method of examples 45-69 or any combination thereof,wherein determining that the fault exists includes determining that ashort circuit exists between a cathode pad of the first pair of pads anda cathode pad of the second pair of pads.

Example 71. The method of examples 45-70 or any combination thereof,further including controlling a connection network to connect an ADC toa terminal of the first LED or a terminal of the second LED one-at-atime.

Example 72. A device includes a built-in self-test for detecting a faultbetween a first LED and a second LED of a set of LEDs includes a set ofpads including a first pair of pads configured to connect to the firstLED and a second pair of pads configured to connect to the second LED.The device also includes a set of drivers, wherein each driver of theset of drivers includes a pass device configured to electricallyconnected to a respective pad of the set of pads, and wherein eachdriver of the set of drivers is configured to control whether arespective LED of the set of LEDs is turned on or off. The devicefurther includes processing circuitry configured to cause a first driverof a set of drivers to turn on a first respective pass device and causea second driver of the set of drivers to control a second respectivepass device to turn off the second LED. The processing circuitry isfurther configured to determine a first forward voltage across the firstLED, determine a second forward voltage across the second LED, anddetermine that a fault exists between a first LED and a second LED ofthe set of LEDs based on the first and second forward voltages.

Example 73. The device of example 72, further including a sense resistorconfigured to be connected to the first anode pad and to the firstcathode pad.

Example 74. The device of example 72 or example 73, further including adifferential ADC configured to measure the voltage across a senseresistor.

Example 75. The device of examples 72-74 or any combination thereof,further including a connection network configured to connect to thefirst anode pad and the first cathode pad.

Example 76. The device of examples 72-75 or any combination thereof,further including a memory configured to store a trimming coefficientfor a sense resistor.

Example 77. The device of examples 72-76 or any combination thereof,further including a memory configured to store a minimum value, amaximum value, and a mean value of voltage differences across each LED,each pair of pads, and/or each driver.

Example 78. The device of examples 72-77 or any combination thereof,wherein the processing circuitry includes a state machine configured tocause the set of drivers to turn on and off pass devices one by one aspart of a self test.

Example 79. The device of examples 72-78 or any combination thereof,wherein the processing circuitry is configured to perform the method ofexamples 45-71 or any combination thereof.

Example 80. A device includes a computer-readable medium havingexecutable instructions stored thereon, configured to be executable byprocessing circuitry for causing the processing circuitry to perform themethod of examples 45-71 or any combination thereof.

Example 81. A system includes means for performing the method ofexamples 45-71 or any combination thereof.

Various examples of the disclosure have been described. Any combinationof the described systems, operations, or functions is contemplated.These and other examples are within the scope of the following claims.

What is claimed is:
 1. A device including a built-in self-test for detecting a fault between a first light emitting diode (LED) and a second LED of a set of LEDs, the device comprising: a set of pads comprising: a first pair of pads configured to connect to the first LED; a second pair of pads configured to connect to the second LED; a set of drivers, wherein each driver of the set of drivers includes a pass device configured to electrically connected to a respective pad of the set of pads, and wherein each driver of the set of drivers is configured to control whether a respective LED of the set of LEDs is turned on or off; and processing circuitry configured to: cause a first driver to control a first respective pass device to turn on the first LED; cause a second driver to control a second respective pass device to turn off the second LED; determine a first forward voltage across the first LED; determine a second forward voltage across the second LED; and determine whether a fault exists between the first LED and the second LED based on the first and second forward voltages.
 2. The device of claim 1, wherein the processing circuitry is further configured to: determine whether the first forward voltage is outside of a first acceptable range for activated LEDs; and determine whether the second forward voltage is outside of a second acceptable range for deactivated LEDs, wherein the processing circuitry is configured to determine that the fault exists in response to: determining that the first forward voltage is outside of the first acceptable range; or determining that the second forward voltage is outside of the second acceptable range.
 3. The device of claim 2, wherein the processing circuitry is configured to determine whether the first forward voltage is outside of the first acceptable range by determining that the first forward voltage is less than a first threshold level for activated LEDs.
 4. The device of claim 2, wherein the processing circuitry is configured to determine whether the second forward voltage is outside of the second acceptable range by determining that the second forward voltage is greater than a second threshold level for deactivated LEDs.
 5. The device of claim 1, further comprising a differential analog-to-digital converter (ADC) configured to: receive, at a first input of the differential ADC, a first signal indicating a voltage level at an anode pad of the first pair of pads; receive, at a second input of the differential ADC, a second signal indicating a voltage level at a cathode pad of the first pair of pads LED; and output a digital number indicating the first forward voltage across the first LED to the processing circuitry.
 6. The device of claim 5, wherein the processing circuitry is further configured to: cause the first input of the differential ADC to receive a third signal indicating a voltage level at an anode pad of the second pair of pads instead of receiving the first signal; and cause the second input of the differential ADC to receive a fourth signal indicating a voltage level at a cathode pad of the second pair of pads instead of receiving the second signal.
 7. The device of claim 1, wherein the processing circuitry is further configured to increment a counter in response to determining that the fault exists between the first LED and the second LED.
 8. The device of claim 7, wherein the processing circuitry is further configured to output a value of the counter to an extremal device via a diagnostic interface.
 9. The device of claim 1, further comprising a memory, wherein the processing circuitry is further configured to store, to the memory, a position of the first LED and a position for the second LED in response to determining that the fault exists between the first LED and the second LED.
 10. The device of claim 9, wherein the processing circuitry is further configured to output the position of the first LED and the position of the second LED to an external device via a diagnostic interface.
 11. The device of claim 1, wherein the processing circuitry is further configured to cause the set of drivers to: turn on a first subset of the set of LEDs; and turn off a second subset of the set of LEDs, wherein the first and second subsets form a chessboard pattern.
 12. The device of claim 1, wherein the processing circuitry is configured to receive a signal from an external device via a diagnostic interface, and wherein the processing circuitry is configured to determine whether the fault exists between the first LED and the second LED in response to receiving the signal from the extremal device.
 13. The device of claim 1, wherein the processing circuitry is configured to determine whether the fault exists between the first LED and the second LED during a startup phase of the device.
 14. The device of claim 1, wherein the first LED is adjacent to the second LED.
 15. The device of claim 1, wherein the processing circuitry is configured to determine that the fault exists by determining that a short circuit exists between a cathode pad of the first pair of pads and a cathode pad of the second pair of pads.
 16. A method for a built-in self-test to detect a fault between a first LED and a second LED of a set of LEDs, the method comprising: causing a first driver of a set of drivers to control a first respective pass device to turn on the first LED, wherein each driver of the set of drivers includes a pass device electrically connected to a respective LED of the set of LEDs, and wherein each driver of the set of drivers is configured to control whether a respective LED of the set of LEDs is turned on or off; causing a second driver of the set of drivers to control a second respective pass device to turn off the second LED; determining a first forward voltage across the first LED; determining a second forward voltage across the second LED; and determining that a fault exists between a first LED and a second LED of the set of LEDs based on the first and second forward voltages.
 17. The method of claim 16, wherein the processing circuitry is further configured to: determine whether the first forward voltage is outside of a first acceptable range for activated LEDs; and determine whether the second forward voltage is outside of a second acceptable range for deactivated LEDs, wherein the processing circuitry is configured to determine that the fault exists in response to: determining that the first forward voltage is outside of the first acceptable range; or determining that the second forward voltage is outside of the second acceptable range.
 18. The method of claim 16, further comprising: turning on a first subset of the set of LEDs; and turning off a second subset of the set of LEDs, wherein the first and second subsets form a chessboard pattern.
 19. A built-in self-test device comprising a computer-readable medium having executable instructions stored thereon, configured to be executable by processing circuitry for causing the processing circuitry to: cause a first driver of a set of drivers to control a first respective pass device to turn on a first LED of a set of LEDs, wherein each driver of the set of drivers includes a pass device configured to electrically connect to a respective pad of a set of pads LEDs, wherein a first pair of pads is configured to connect to the first LED, and wherein each driver of the set of drivers is configured to control whether a respective LED of the set of LEDs is turned on or off; cause a second driver of the set of drivers to control a second respective pass device to turn off a second LED of the set of LEDs, wherein a second pair of pads is configured to connect to the second LED; determine a first forward voltage across the first LED; determine a second forward voltage across the second LED; and determine that a fault exists between a first LED and a second LED of the set of LEDs based on the first and second forward voltages.
 20. The built-in self-test of claim 19, further comprising instructions configured to cause the processing circuitry to: determine whether the first forward voltage is outside of a first acceptable range for activated LEDs; and determine whether the second forward voltage is outside of a second acceptable range for deactivated LEDs, wherein instructions to determine that the fault exists comprise instructions to determine that the fault exists in response to: determining that the first forward voltage is outside of the first acceptable range; or determining that the second forward voltage is outside of the second acceptable range. 